Really? So riding a hill slowly and placing very little stress on your CV system is better than riding hard on the flat and stressing your CV system? Stress which leads to adaptations in an area the OP admitted to lacking ie:fitness???? Do you not know how the body works?
Ach, I can't be bothered with the rest. You seem to be struggling with the point.
Riding up a hill slowly can yield the same CV effort as riding fast on the flat. You obviously have a reasonable understanding of how the body works so you can accept that.
I'm quite obviously not advocating Granny Ring and 3mph up hill now am I? I'm comparing like with like.
- I can be broken, dying on my handlebars at 10mph uphill. The Speed is slow, the effort is high.
- I can be happy and whistling, at 20mph on the flat. The speed is high, the effort is low.
To me, the best prescription for a person who's symptoms are 'Can't climb hills' is... climb more hills. I'm not going to send them out thrashing themselves on the flat. I know they could get the same CV gains (and indeed acompanying weight loss and/or muscle gains) from doing so, it's just a lot more work (in terms of how far you will travel in that time, interactions with traffic etc).
1 hill, 3.2 Miles long, 6.7%, 25 Minutes to climb at 80% effort.
1 ride. 25 Minutes. 80% effort. 10.5 Miles covered (@ an arbitrary 25mph average).
It's a huge ask for a beginner to ask them to flog themselves for 10 Miles, it just is. Generally, they'll back off at the first sight of tiredness. This does not happen on hills. You can't roll a little, there is no coasting, there is getting to the top, or stopping.